1. Field of the Invention
The present invention relates to an EGR control apparatus and method for an internal combustion engine for burning an air-fuel mixture with self ignition while auxiliary using a spark ignition.
2. Description of the Prior Art
Conventionally, as an EGR control apparatus for an internal combustion engine, one described, for example, in Laid-open Japanese Patent Application No. 11-210539 is known. This internal combustion engine is of a type which can be operated by switching between a self ignition combustion mode for burning an air-fuel mixture with self ignition while auxiliary using a spark ignition and a flame propagation combustion mode for burning an air-fuel mixture with flame propagation using a spark ignition. This internal combustion engine comprises an EGR control valve for controlling the amount of exhaust gases recirculated from an exhaust passage to an intake passage, and a valve timing mechanism for changing opening/closing timings of an intake air valve by changing the phase of an intake cam shaft with respect to a crank shaft. The EGR control apparatus also comprises an intake air temperature sensor for detecting an intake air temperature, and calculates a gas temperature TE which is the temperature of an air-fuel mixture within a cylinder, i.e., an unburnt gas at the timing of ignition.
In a control process shown in FIG. 7 of this EGR control apparatus, it is determined, in accordance with an engine load and an engine rotational speed, whether an operating condition of the internal combustion engine is in an operation range 1 in which the flame propagation combustion mode can be executed, or in operation ranges 2, 3 in which the self ignition combustion mode can be executed (step 100), and a target temperature TEO and a gas temperature TE are calculated when it is determined that the internal combustion engine is in the operation ranges 2, 3 in which the self ignition combustion mode can be executed (steps 102, 103). Then, when TE>TEO is established, the opening of the EGR control valve is controlled toward a decreasing side by a predetermined value in order to reduce the gas temperature TE (step 105). On the other hand, when TE≦TEO is established, the opening of the EGR control valve is controlled toward an increasing side by a predetermined value in order to increase the gas temperature TE (step 106). Also, in this EGR control apparatus, when the operating condition of the internal combustion engine is in the operation range 3 of high loads, where it is difficult to increase the gas temperature TE to a predetermined range with recirculated gases, the flame propagation combustion mode is executed with a normal spark ignition (Paragraphs [0037], [0038], [0048]).
According to the conventional EGR control apparatus for the internal combustion engine, a high-temperature recirculated gas is recirculated into a combustion chamber through the EGR control valve in order to increase the gas temperature TE to a predetermined range with the EGR control, so that when the normal flame propagation combustion mode is executed while the operating condition of the internal combustion engine is in the operation range 3 of high loads, a reduction in combustion temperature by the recirculated gas cannot be made for purposes of avoiding knocking. As a result, in order to avoid knocking, an ignition timing retard control and the like are obliged to be executed, possibly resulting in a reduction in combustion efficiency, engine output, and drivability.